1. Field of the Invention
The present invention relates to synthesis of digitized images and, more particularly, relates to an image synthesizing apparatus suitable for recording an arbitrary-form foreground image superposed on another background image.
2. Description of the Prior Art
Recently, an image processing system designed to receive, edit and record digitized natural images has been used in work stations and the like. For example, a system of this type is introduced in the reading, "Report on CD-ROM, CD-I Development Field", Nikkei New Media.Technical Forefront Report 1, Nikkei McGraw-Hill Inc., pages 36 to 45, May 1987. The system introduced in the afore-mentioned reading is constituted by a 16-bit micro-processor as a main component, a frame buffer (display memory) of 24-bit depth (8-bit depth in each of R, G and B), a video camera for fetching natural images, an A/D converter, a monitor for confirming the contents of the frame buffer, a tablet or terminal for inputting coordinates and characters, a work memory, and an external storage. The natural images thus fetched can be subject to editing/processing operations, such as removal of noise, change of color tone (saturation and lightness), change of contrast, drawing of outlines, enlargement or reduction of scale, mask synthesis and the like. The term "mask synthesis" used herein means image synthesis which is performed by superposing part of an image on another background image. In this regard, in the aforementioned prior art example, a mask memory is arranged in parallel to the frame buffer (display memory) so that the background image can be combined with the image extracted by masking, with reference to the contents of the mask memory. In general, the reference to the mask memory is carried out by software.
On the other hand, Japanese Patent Unexamined Publication No. 59-45567 discloses a method in which image synthesis is sped up by circuit control on the basis of the judgment from the contents of the mask memory whether data are to be transferred to the corresponding frame buffer or not.
According to the aforementioned prior art, the mask memory must be arranged in parallel to the frame buffer, so that there arises a problem in that the circuit is enlarged in scale. Further, in the case where the reference to the mask memory is carried out by software, a large amount of time is required for image synthesis. On the other hand, in the case where high-speed synthesis is made by circuit control, the mask memory must have a sufficiently short access time compared with the frame buffer. Accordingly, the circuit is apt to be expensive while the circuit scale is apt to be enlarged. Further, in the case where an image to be superposed is fetched on the background preliminarily formed as a fixed uniform color on the assumption of mask synthesis as called "chromakey", an additional procedure comprising the steps of preparing a mask pattern by reference to the aforementioned image and recording it in the mask memory is required. There arises therefore a problem in that a long time is required for image synthesis.